Hi abbygrant,
While a mattress that is too firm can certainly cause shoulder issues … there can be different types of shoulder issues that can each have different or even multiple causes and some of the suggestions that both I and Sam made may be well worth exploring.
While it’s not possible to “diagnose” mattress comfort issues on a forum with any certainty because there are too many unique unknowns, variables, and complexities involved that can affect how each person sleeps on a mattress in terms of “comfort” and PPP (Posture and alignment, Pressure relief, and your Personal preferences) or any “symptoms” they experience … there is more about the most common symptoms that people may experience when they sleep on a mattress and the most likely (although not the only) reasons for them in post #2 here.
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
These posts are the “tools” that can help with the analysis, detective work, or trial and error that may be necessary to help you learn your body’s language and “translate” what your body is trying to tell you so you can identify the types of changes that have the best chance of reducing or eliminating any “symptoms” you are experiencing … at least to the degree that any symptoms are from your mattress rather than the result of any other circumstances or pre-existing issues you may have that may not be connected to a mattress.
A more detailed conversation with Novosbed on the phone may also be helpful because you can cover more ground and deal with more detailed issues much more effectively in a more “real time and interactive” phone conversation than you can with written communications (see post #4 here)
Phoenix